Antalek Matthew, Pace Elisabetta, Hedman Britt, Hodgson Keith O, Chillemi Giovanni, Benfatto Maurizio, Sarangi Ritimukta, Frank Patrick
Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, USA.
Laboratori Nazionali di Frascati-INFN, P.O. Box 13, 00044 Frascati, Italy.
J Chem Phys. 2016 Jul 28;145(4):044318. doi: 10.1063/1.4959589.
Three-dimensional models for the aqueous solvation structures of chloride, bromide, and iodide are reported. K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near edge (MXAN) analyses found well-defined single shell solvation spheres for bromide and iodide. However, dissolved chloride proved structurally distinct, with two solvation shells needed to explain its strikingly different X-ray absorption near edge structure (XANES) spectrum. Final solvation models were as follows: iodide, 8 water molecules at 3.60 ± 0.13 Å and bromide, 8 water molecules at 3.40 ± 0.14 Å, while chloride solvation included 7 water molecules at 3.15 ± 0.10 Å, and a second shell of 7 water molecules at 4.14 ± 0.30 Å. Each of the three derived solvation shells is approximately uniformly disposed about the halides, with no global asymmetry. Time-dependent density functional theory calculations simulating the chloride XANES spectra following from alternative solvation spheres revealed surprising sensitivity of the electronic state to 6-, 7-, or 8-coordination, implying a strongly bounded phase space for the correct structure during an MXAN fit. MXAN analysis further showed that the asymmetric solvation predicted from molecular dynamics simulations using halide polarization can play no significant part in bulk solvation. Classical molecular dynamics used to explore chloride solvation found a 7-water solvation shell at 3.12 (-0.04/+0.3) Å, supporting the experimental result. These experiments provide the first fully three-dimensional structures presenting to atomic resolution the aqueous solvation spheres of the larger halide ions.
报道了氯离子、溴离子和碘离子水合溶剂化结构的三维模型。K边扩展X射线吸收精细结构（EXAFS）和Minuit X射线吸收近边（MXAN）分析发现，溴离子和碘离子有明确的单壳溶剂化球。然而，溶解的氯离子在结构上有所不同，需要两个溶剂化壳来解释其显著不同的X射线吸收近边结构（XANES）光谱。最终的溶剂化模型如下：碘离子，8个水分子，距离为3.60±0.13 Å；溴离子，8个水分子，距离为3.40±0.14 Å；而氯离子溶剂化包括7个水分子，距离为3.15±0.10 Å，以及第二个由7个水分子组成的壳层，距离为4.14±0.30 Å。三个推导的溶剂化壳层中的每一个都大致均匀地分布在卤化物周围，没有整体不对称性。随时间变化的密度泛函理论计算模拟了替代溶剂化球后的氯离子XANES光谱，结果表明电子态对6、7或8配位具有惊人的敏感性，这意味着在MXAN拟合过程中正确结构的相空间受到强烈限制。MXAN分析进一步表明，使用卤化物极化的分子动力学模拟预测的不对称溶剂化在整体溶剂化中不起重要作用。用于探索氯离子溶剂化的经典分子动力学发现，在3.12（-0.04/+0.3）Å处有一个7水溶剂化壳层，支持了实验结果。这些实验提供了首个原子分辨率的完整三维结构，展示了较大卤离子的水合溶剂化球。
